1. Field of the Invention
This invention relates to the field of variable frequency oscillators.
2. Background Art
Many integrated circuit applications utilize a variable frequency oscillator (VFO) to provide timing signals to the circuit. In some applications, the VFO is used with a phase locked loop (PLL) as, for example, in connection with a data synchronizer. The center frequency of a VFO is set by controlling the charging currents to the timing capacitor of a voltage controlled oscillator (VCO). In the prior art, the charging currents were made proportional to, and thus controlled by, the VCO control voltage Vc.
In PLL applications, the gain of the VFO should be independent of the control voltage Vc. In this manner, the open loop gain of the VFO will not vary with changes in Vc. This reduces loop stability problems caused when the gain is dependent on the control voltage.
In certain instances, it is desired to utilize the VCO charging currents to control a plurality of timing signals in addition to controlling the center frequency of the VFO. Under such circumstances, however, the control voltage Vc cannot be used to control the charging currents as well as additional timing signals without the use of a plurality of external control resistors. A single resistor solution is not possible when the timing currents are controlled by the control voltage Vc.
Therefore, it is desired to provide a VFO in which a single external resistor is utilized to set the center frequency and a plurality of timing signals.
It is a further object of the present invention to provide a VFO in which a single external resistor is utilized to control center frequency and the VFO gain (timing constant) is independent of control voltage.
It is still another object of the present invention to provide a VFO whose center frequency can be set by a single external resistor which can be changed for a wide range of timing requirements.
It is yet another object of the present invention to provide a VFO utilizing a single external resistor for better setting accuracy.